The present invention concerns automatic and semiautomatic focussing systems for photographic still and motion-picture cameras.
In particular, the present invention concerns automatic and semiautomatic focussing systems of the unidirectional type. In a unidirectional automatic focussing system, the subject-distance setting of the camera objective is always at one of the two extremes of the total subject-distance range at the start of a focussing operation, e.g., at minimum subject-distance setting, and then is moved by a drive spring or electric drive motor towards the other extreme setting, the change of subject-distance setting being discontinued when the correct subject-distance setting has been reached. In such unidirectional automatic focussing systems, the drive used for the objective, whether a drive spring or an electric motor, does not adjust the subject-distance setting bidirectionally, but instead always, as just stated, proceeding from one extreme setting to the correct setting. Even when the focussing system is provided with an adjusting motor of bidirectional operation, a switchover to what is in effect unidirectional operation may be implemented. For example, commonly owned copending application Ser. No. 20,240 filed Mar. 13, 1979, now U.S. Pat. No. 4,265,528, discloses a system in which the subject-distance setting is automatically returned to an extreme value at the end of each exposure or series of exposures; accordingly, that system, too, operates essentially unidirectionally. Indeed, an automatic return to an extreme subject-distance setting can be performed even in systems where the subject-distance setting is manually adjusted; in that event, as the user moves the camera objective from one extreme setting towards the other extreme setting, an indicator lamp can light up, or the like, to command termination of the unidirectional movement.
For various reasons, in automatic unidirectional focussing systems, the circuitry used to ascertain the correct subject-distance setting has been mainly of the travel-time measurement type, involving measurement of the time of travel of radiation emitted from the camera to the subject, and reflected from the subject back to the camera.